Modern computers such as desktop computers, server computers, and so forth, can be powered by plugging the computers into external power sources, such as wall outlets that supply alternating current (AC) power, When computers are not in use, power saving features are usually provided to automatically transition the computers to a low power mode (such as a hibernation mode). Alternatively, users who do not plan to use their computers for some amount of time may decide to shut off the computers.
However, even when computers are shut off or in hibernation mode, such computers can still draw power from external power sources, which can be wasteful of power resources. The cumulative effect of potentially millions of computers continuing to draw power even when such computers are shut off can result in substantial energy waste.